For the machining of several non-ferrous alloys like brass and magnesium, diamond is the best tool material, whereas cubic boron nitride (cBN) is very well suited for machining hardened steel, chilled cast iron, and cast iron at elevated speeds.
The cutting inserts used in such machining operations are either so-called PCD (polycrystalline diamond compact) or PcBN (polycrystalline cubic boron nitride compact) inserts, comprising a cemented carbide body with a diamond or cBN layer applied at such high temperature and pressure where the diamond or cBN is the stable phase or the inserts are provided with diamond or cBN bodies in at least one corner or along an edge generally fastened by brazing.
The method of manufacturing such inserts has been described in, e.g., U.S. Pat. Nos. 3,745,623; 3,743,489; 4,525,179; 5,589,621 and 5,676,496; and such descriptions are incorporated herein by reference.
Many inserts used for machining have holes in the center to ensure the best possible attachment to the tool holder. In the case of brazed inserts, this can be achieved by brazing a PCD/CBN tip to a body that already has a hole in it. Otherwise, the hole has to be cut afterwards, which is costly since cutting usually involves plunge EDM ("electro discharge machining") to start a small hole followed by wire cutting to make the actual size hole. In this way only straight-sided holes can be made. Holes cannot be cut to satisfy, e.g., holes with a chamfer section.
In a brazing process, a tungsten carbide body undergoes a thermal cycle and a PCD/CBN tip is bonded to the body by a brazing agent to form one cutting edge. To obtain multiple cutting edges, multiple thermal cycles are necessary. Because multiple thermal cycles tend to deteriorate the inserts, it is not desirable to apply multiple cutting edges by the brazing technique. Although multiple cutting edges may be made by the high pressure/high temperature sintering process on a carbide insert, post-sintering hole-drilling is necessary to form a hole in the carbide insert, which is time-consuming and expensive.
For the foregoing reasons, there exists a need for a hole-making process which is cost-effective and is capable of producing multiple cutting edges. Furthermore, it is desirable that such a process not subject inserts to multiple thermal cycles.